U.S. patent number 6,318,247 [Application Number 09/054,276] was granted by the patent office on 2001-11-20 for appliance for preparation of heated and stirred beverages and foods.
This patent grant is currently assigned to Sunbeam Products, Inc.. Invention is credited to David Di Nunzio, Timothy E. Lint.
United States Patent |
6,318,247 |
Di Nunzio , et al. |
November 20, 2001 |
Appliance for preparation of heated and stirred beverages and
foods
Abstract
An appliance for preparation of heated and stirred beverages and
foods has a vessel and a base, with a mixing element rotationally
driven within the vessel by magnetic coupling with a driving magnet
array attached to an electric motor and covered by a cap. The
mixing element is mounted on a rolling contact bearing assembly
which fits upon a pin which extends from the cap through an opening
in the vessel bottom. The vessel bottom is preferably a cast metal
piece with internal cavities in which one or more heating elements
are mounted whereby a liquid in the vessel is simultaneously heated
and stirred or whipped. Electrical power to a mixing motor and
heating elements is controlled either by a manually resettable
thermostat, or by a self-latching relay controlled switching
circuit which includes a blocking diode to supply rectified AC
current to the motor for low torque start ups which will not
decouple the magnetic flux drive of the mixing element.
Inventors: |
Di Nunzio; David (Mentor,
OH), Lint; Timothy E. (Parma, OH) |
Assignee: |
Sunbeam Products, Inc. (Boca
Raton, FL)
|
Family
ID: |
21989940 |
Appl.
No.: |
09/054,276 |
Filed: |
April 2, 1998 |
Current U.S.
Class: |
99/348; 366/146;
366/274; 99/287; 99/510 |
Current CPC
Class: |
A47J
27/004 (20130101); A47J 43/042 (20130101); A47J
43/0465 (20130101) |
Current International
Class: |
A47J
43/04 (20060101); A47J 43/042 (20060101); A47J
43/046 (20060101); A47J 27/00 (20060101); A47J
043/04 (); A47J 037/00 (); B01F 015/06 (); B01F
013/04 () |
Field of
Search: |
;99/348,510,287,275
;366/273,274,146,205 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Alexander; Reginald L.
Attorney, Agent or Firm: Kramer, Levin, Naftalis &
Frankel LLP
Claims
What is claimed is:
1. A device for preparing heated and mixed beverage, the device
comprising:
a base underneath a vessel adapted to receive a quantity of liquid
to be prepared as a heated beverage,
a vessel bottom supported by the base and having a central opening
in the vessel bottom,
a motor connected to a shaft, the motor supported by the base to
orient the shaft generally orthogonal to the opening in the vessel
bottom, the motor shaft supporting a driving magnet array and
operative to rotate the driving magnet array, a cap over the
driving magnet array, the cap having a cap pin in general alignment
with the motor shaft,
a mixing element attached to a driven magnet array and mounted for
rotation upon the cap pin, rotation of the mixing element upon the
cap pin induced by magnetic coupling with rotation of the driving
magnet array.
2. The device of claim 1 further comprising a driving magnet array
housing attached to the motor shaft and having cavities in which
magnets of the driving magnet array are located.
3. The device of claim 1 further comprising a motor mount attached
to the motor and attached to the vessel bottom.
4. The device of claim 3 wherein the cap over the driving magnet
array includes a vertical wall which stands on the motor mount.
5. The device of claim 4 wherein a vertical extent of the vertical
wall of the cap is greater than a length of the motor shaft,
whereby a top wall of the cap is positioned over the driving magnet
array.
6. The device of claim 1 further comprising a seal between the cap
and the perimeter of the opening in the vessel bottom.
7. The device of claim 1 wherein the mixing element is attached to
a driven magnet array housing which supports magnets of the driven
magnet array.
8. The device of claim 1 wherein the mixing element comprises a
vertical wall which substantially surrounds the driven magnet array
and attaches to the driven magnet array housing.
9. The device of claim 1 wherein the mixing element comprises at
least two mixing blades.
10. The device of claim 1 wherein the mixing element comprises a
handle and two laterally opposed mixing blades which are smaller in
size than the handle.
11. The device of claim 1 wherein the mixing element has only two
mixing blades which are opposed one hundred and eighty degrees.
12. The device of claim 1 wherein the mixing element is mounted
upon a rolling contact bearing assembly which is attached to the
cap pin.
13. The device of claim 1 further comprising a vessel wall and base
wall which are substantially contiguous on an exterior of the
device.
14. The device of claim 13 wherein the vessel wall comprises a
notch in which an upper edge of the base wall is received, whereby
the vessel is at least partially supported by the base.
15. The device of claim 13 wherein the vessel wall comprises a
lower segment configured to fit inside of the base wall and having
an lip adapted to support a flange of the vessel bottom.
16. The device of claim 15 further comprising a seal between the
lip of the vessel wall and the vessel bottom flange.
17. The device of claim 1 further comprising at least one boss in
the base which is attachable to the vessel bottom.
18. The device of claim 1 wherein the cap is made of a non-ferrous
material.
19. The device of claim 1 further comprising at least one heating
element in thermal contact with the vessel bottom.
20. The device of claim 1 further comprising two heating elements
in the vessel bottom, wherein a wattage rating of one heating
element is substantially less than a wattage rating of the other
heating element.
21. The device of claim 1 further comprising an electrical power
and control circuit operative to supply and control electrical
power to the motor and to heating elements in thermal contact with
the vessel bottom, the circuit including a connection to a 120 volt
AC power supply, two resistor heating elements wherein a wattage
rating of one heating element is substantially less than a wattage
rating of another heating element, a full wave bridge rectifier
having four diodes and connected in parallel with the heating
element of lesser wattage and connected to an electric motor, and a
manually resettable thermostat.
22. The device of claim 1 further comprising an electrical power
and control circuit operative to supply and control electrical
power to the motor and to heating elements in thermal contact with
the vessel bottom, the circuit including a connection to a 120 volt
AC power supply in series with an automatic reset thermostat and
two resistor heating elements, one of the heating elements having a
wattage rating less than a wattage rating of the other heating
element, a full wave bridge rectifier having four diodes and
connected in parallel across the heating element of lesser wattage
and connected to an electric motor, and a self-latching
relay-controlled circuit connected in parallel to the motor, the
self-latching relay controlled circuit including a relay control
switch connected in parallel with a free wheeling diode, a
filtering capacitor, and a current limiting resistor and an LED,
the relay connected in series to a collector of a transistor, a
base of the transistor connector to receive direct current from the
bridge through a blocking diode and a delay resistor, the base
further connected to a first pole of a switch which is in parallel
with a delay capacitor 126, a second pole of the switch connected
to the bridge.
23. The device of claim 1 further comprising a lid adapted to cover
a top opening to the vessel.
24. The device of claim 21 wherein the lid comprises a gripping
structure and at least one flange configured to fit within the
vessel wall.
25. The device of claim 1 wherein the vessel comprises a pouring
spout.
26. The device of claim 1 wherein the vessel bottom is generally
concave.
27. A mixing apparatus comprising:
a vessel for receiving at least one substance to be heated and
stirred, the vessel having a vessel bottom with a central
opening,
a base which supports the vessel and the vessel bottom, and a
motor, the motor having a shaft which is generally axially aligned
with the central opening in the vessel bottom,
a driving magnet array attached to the motor shaft,
a cap surrounding the driving magnet array,
a cap pin extending from the cap and generally axially aligned with
the motor shaft and with the central opening in the vessel
bottom,
a mixing element assembly having an array of driven magnets and an
opening for receiving a distal end of the cap pin, the driven
magnet array of the mixing element assembly being magnetically
coupled with the driving magnet array attached to the motor shaft,
whereby a mixing element of the mixing element assembly is rotated
upon rotation of the driving magnet array attached to the motor
shaft.
28. The apparatus of claim 27 further comprising at least one
heating element in thermal contact with the vessel bottom.
29. The apparatus of claim 27 comprising first and second heating
elements in thermal contact with the vessel bottom, the first
heating element having a wattage rating substantially less than the
second heating element.
30. The apparatus of claim 27 wherein the first and second heating
elements are located in cavities in the vessel bottom, and the
first heating element is located radially outside of the second
heating element.
31. The apparatus of claim 27 wherein an electrical connection to
the motor is in parallel with the first heating element.
32. The apparatus of claim 27 further comprising a seal between the
cap and the opening in the vessel bottom.
33. The apparatus of claim 27 wherein the mixing element assembly
comprises a driven magnet array housing having a cavity for
receiving magnets of the driven magnet array, and a mixing element
attached to the driven magnet array housing, the mixing element
having at least one blade which extends away from the driven magnet
array.
34. The apparatus of claim 27 further comprising an electrical
power and control circuit operatively connected to the motor and
the heating element, the circuit comprising a relay activated power
control circuit in parallel with a resistor heating element.
Description
FIELD OF THE INVENTION
The present invention pertains generally to devices that heat and
mix, including food preparation appliances and, more particularly,
to appliances for preparing food and beverages, including food and
beverages which are heated and mixed.
BACKGROUND OF THE INVENTION
Many different types of media mixing devices have been devised,
including industrial, chemical and food and beverage mixers and
preparation appliances. Blenders have a motor-driven blade mounted
at the bottom of a pitcher to chop, stir, whip or blend solids or
liquids loaded into the pitcher. Conventionally, the blender blade
is mounted directly on a motor shaft, and is removable to allow the
pitcher to be removed from a base in which the motor is housed. The
motor is electrically driven at various selectable speeds. The
necessity to remove the blade, or to mechanically disengage it from
the driving shaft prior to removal of the pitcher is
problematic.
Some attempts have been made to induce rotation of a mixing element
or impeller by magnetic coupling with a mechanically driven magnet
array. U.S. Pat. No. 3,554,497 describes a prior art stirring
device which has a motor driven magnet in a housing, magnetically
coupled to a stirring magnet inside of a vessel. The patent also
describes magnetic field induced rotation of a stirring member in a
vessel by electronically controlled driving of a fixed magnet array
within a housing on which the vessel rests. Described for use in
chemical laboratories and being explosion proof, the lack of any
mechanical registry of the mixing element allows for irregular
rotational motion and decoupling of the magnetic drive.
Some blending or stirring machines have also been provided with a
heat source in contact with a vessel or pitcher in which a blade is
mechanically rotated, in order to prepare foods or vegetables which
are both stirred and heated. A heat source is in such devices
located at the bottom of the vessel has the disadvantage of
providing rather poor heat distribution through the liquid or food
in the pitcher or vessel.
SUMMARY OF THE INVENTION
The present invention provides a food preparation device for
stirring and heating liquids or combinations of liquids and solids
such as beverages or soups. In accordance with one aspect, the
device includes a base which supports a vessel for receiving a
quantity of liquid to be prepared as a heated beverage, a vessel
bottom having a heating element, and a central opening in the
vessel bottom, a motor connected to a shaft, the motor supported by
the heater casting to orient the shaft generally orthogonal to the
opening in the vessel bottom, the motor shaft supporting a driving
magnet array and operative to rotate the magnet array, a cap over
of the driving magnet array, the cap having a cap pin in general
alignment with the motor shaft, and a mixing element attached to a
driven magnet array and mounted for rotation upon the cap pin,
rotation of the mixing element upon the cap pin induced by rotation
of the driving magnet array by the motor.
These and other aspects of the invention are here in described in
particularized detail with specific reference to the accompanying
Figures which illustrate representative and preferred embodiments
of the various principles of the invention.
DESCRIPTION OF THE FIGURES
In the accompanying Figures:
FIG. 1 is a cross-sectional elevational view of a food preparation
device in accordance with the present invention;
FIGS. 2A and 2B are cross-sectional elevational views of alternate
embodiments of the food preparation device of the present
invention;
FIG. 3A is a top view, and FIGS. 3B-3C are cross-sectional views of
a ball bearing assembly in combination with a mixing element in
accordance with the present invention, and
FIGS. 4A and 4B are schematic drawings of a self-latching
electrical control circuit for use with the appliance of the
present invention.
FIG. 4C is a schematic representation of the control circuit shown
for use with the appliance of the present invention as shown in
FIG. 2A.
DETAILED DESCRIPTION OF PREFERRED AND ALTERNATE EMBODIMENTS
As shown in FIGS. 1, 2A and 2B, the invention is an appliance,
indicated entirely at 10, which includes a base section 12 and an
upper section 14 in the form of a vessel or reservoir for receiving
liquid. The upper section or vessel 14 includes side walls 16, a
handle 18 which may be integrally formed with the side walls, and a
pouring spout 20 which may also be integrally formed with the side
walls. A removable lid 22 covers a top opening of the upper section
14 to provide access to the vessel. The lid 22 may have an
integrally formed handle or gripping structure 24 and interference
fit flanges 26 which extend into the vessel when the lid is in
position over the opening of the vessel.
The base section 12 also has side walls 30 which are substantially
flush with the vessel side walls 16. An upper edge 32 of the base
section side walls 30 are engaged in an offset groove 28 in the
vessel side walls 16 whereby the upper section 14 is supported by
the base section 12 about its entire perimeter. Below the offset
groove 28, the vessel side walls 16 are interior to the base
section side walls 30. The base and upper section side walls may be
of any suitable horizontal cross-sectional configuration such as
generally circular, oval or other. The vessel side walls 16
terminate at a lower end inside of the base section side walls in a
generally perimetric flange 34. A seal 36 is supported by the upper
interior surface of the flange 34. On the seal rests a perimetric
flange 38 of a vessel bottom 40. In the embodiment of the
invention, the vessel bottom 40 is preferably constructed of a
metal having good thermal conductivity, and cast with interior
cavities 42 in which one or two or more heating elements 43, such
as resistive elements in magnesium as well known in the art, are
mounted. In a preferred embodiment, the vessel bottom includes two
separate cavities 42a and 42b which pass substantially entirely
through the vessel bottom, and one of the corresponding heating
elements, preferably the inner element 43b, is of much greater
resistance and wattage (such as approximately 500 watts) than the
outer element 43a (such as approximately 100 watts). Other watt
ratings may be employed in accordance with the invention. The use
of two different heating elements of differing wattage ratings, one
being substantially less than the other, provides a lower voltage
tap from the heating element circuit to drive the motor 50, as
further described with reference to FIG. 4.
The vessel bottom 40 is also supported by one or more bosses 44
which extend upward from the bottom wall 13 of the base section 12.
A fastener 45 such as a screw may be attached to the vessel bottom
through the bosses 44.
In a preferred embodiment of the invention, the vessel bottom 40 is
generally circular with a slightly concave upper surface and has
central circular opening, defined by the perimetrical edge 41, so
that the vessel bottom is generally in the form of a ring having an
expansive top surface area. The perimetric flange 38 increases the
top surface area of the vessel bottom for greater heat transfer to
liquid in the vessel. A motor 50 is mounted within the base section
14 so that a shaft 52 which extends from the motor is generally
axially aligned with a center of the opening in the vessel bottom
40. As shown, the motor 50 is attached at an upper end to a motor
mount 54 which is secured to the underside of the vessel bottom 40
by one or more fasteners 53 such as screws. This provides an
integrated unit of the vessel bottom and the motor which can be
easily assembled to the combined base and upper sections by
insertion of the vessel bottom (with the motor attached) into the
vessel and the installation of fasteners 45 through the bottom of
the base section.
Attached to the upper side of the motor mount 54 and over the motor
shaft 52 is a cap 56. The cap 56 has generally vertical side walls
57 which are greater in height than the extent of the shaft 52 from
the motor casing, and a top wall 59 which extends over the entire
upper end of the motor casing. A liquid seal 58 is positioned
between the cap and the edge 41 about the opening in the vessel
bottom. The cap 56 further has a cap pin 60 which is axially
aligned with the motor shaft 52 and extends away from the motor and
extends at least partially through the opening in the vessel
bottom. In other embodiments, the cap shaft may extend entirely
through the opening in the vessel bottom, or may not extend at all
through the opening in the vessel bottom. An important aspect of
the invention is that the cap 56 is made of non-ferrous preferably
plastic material which does not interfere with the magnetic
coupling of the magnet arrays.
A driving magnet array 62 is attached to the motor shaft by a
driving magnet array housing 64 which is preferably in the form of
a generally annular disk having multiple cavities in which driving
magnets 61 are held. A mixing element assembly, generally indicated
at 70, is rotationally mounted upon the cap pin 60. The mixing
element assembly 70 includes a driven magnet array 72 supported by
a driven magnet array housing 74 which is preferably in the form a
generally annular disk having one or more cavities to receive the
driven magnets 73. The driven magnet array housing 74 has a central
aperture 75 in which the cap pin 60 is received. In the simplest
form of the invention, the central aperture 75 of the driven magnet
array housing is a generally cylindrical cavity which receives the
cap pin 60 and allows the housing to freely rotate about the pin by
contact only with the distal end or tip of the pin. In one
embodiment, a single ball bearing 71 is positioned on the tip of
the cap pin, as shown in FIG. 1, and the central aperture 75 of the
housing 74 is placed over the ball bearing to minimize frictional
resistance to rotation of the mixing element.
A mixing element 76 is connected to the driven magnet array housing
74 to rotate therewith. The mixing element includes a base section
77 which may be generally disk shaped, and annular downward
extending wall 78 which mates with the perimeter of the driven
magnet array housing 74, a handle 79, and one or more radial mixing
blades 80 which extend from the upper surface of the base section
77. Any shape, size and number of mixing blades which extend from
the base section 77 are possible within the scope of the invention.
The size, number and shape of the mixing blades are determined
according to such factors as the operating rpm of the motor, the
strength of the magnetic forces between the driving and driven
magnet arrays, and the viscosity of the media placed in the vessel.
The present invention is especially well suited for the preparation
of whipped hot chocolate. The relatively small size of the mixing
element 76 and the mixing blades 79 and 80 in comparison to the
volume of the vessel have been found to produce a high efficiency
mixing and whipping action of liquid such as milk mixed with a
flavoring agent such as cocoa or chocolate, to produce a correct
amount of froth on the top of a liquid within the vessel. The
wattage of the heating elements 43 is optimally selected to
thoroughly heat a liquid within the vessel, and to avoid
overheating which can cause scalding and curdling of milk or other
beverage ingredients. The mixing element assembly 70 may
alternatively be mechanically removably coupled to the motor shaft
52 which would extend through the cap 56 and a suitable seal.
The motor 50 and heating elements 43 are electrically powered by
120 VAC through one of two different types of control circuits
described herein. In one version shown in FIG. 2A, the circuit is
activated by depressing a spring biased pushbutton 51 mounted in an
escutcheon plate 49 in the base wall 30. The pushbutton 51 contacts
a manually resettable thermostat 108, mounted to an arm 47 which
extends from the vessel bottom 40. FIG. 4C illustrates a simplified
schematic for this embodiment, in which power from a standard
supply 200 is series connected to the heating elements 43 through a
manually resettable thermostat 108. A lamp 204 indicates power is
applied, and the motor input voltage is rectified using a full
bridge rectifier circuit 206 connected to the motor 50. Alternative
embodiments of the control circuit are described herein with
respect to FIGS. 2B, 4A and 4B.
In a particular preferred embodiment of the invention, a rolling
contact bearing assembly, indicated generally at 90 and illustrated
in isolation in FIGS. 3A-3C, is installed in the central aperture
75 of the driven magnet array housing 74. The bearing assembly 90
includes an inner ring 92, an outer ring 94, balls 96 therebetween,
radially separated by cages 98. The internal wall 91 of inner ring
92 is frictionally fit with cap pin 60, and the exterior wall
surface 95 of outer ring 94 is frictionally fit or otherwise
secured to the interior wall surfaces of the central aperture 75.
This arrangement gives the mixing element assembly 70 a high degree
of rotational freedom relative to the cap 56, reducing mechanical
drag which works against the magnetic flux rotational driving
force. For beverages, preferred rotational speeds in the
approximate range of 2500 to 3500 rpm are attainable. Other speeds
are possible, dependent upon the size of the motor, the magnets,
and the impeller, and subject to the viscosity of material in the
vessel.
In one embodiment, power to the heating elements 43 and the motor
50 is controlled by a self-latching circuit 100 schematically shown
in FIG. 4A, including the electronics components within the line
box 101 which may be mounted upon a printed circuit board 140
(PCB). The circuit 100 can be connected to a standard 120 VAC power
supply 102, such as by a standard wall outlet and power cord. The
circuit 100 may be made, for example, of discrete components
disposed on a printed circuit board and/or other mounting
arrangement within the base section 12, or with various of the
components integrated into an application specific device. A pair
of contacts 104 and 106 are connected in parallel with each other
and further in series with a thermostatic device 108 and a pair of
thermal permanent cut-off devices 110 and 112. The thermostat 108
may be mounted directly to the vessel bottom 40, as shown in FIG.
2A. Alternatively, the switch and relay contacts 104, 106, and the
thermostat 108 may be mounted directly on a printed circuit board
140 mounted within the base section 12 as shown in FIG. 2B and
further described below. In this circuit it is preferably an
automatic reset type thermostat such as produced by Texas
Instruments. Completing the power circuit are the heaters 43 in
series with the supply 102.
The first set of contacts 104 are normally open as shown and are
controlled by a manually actuated ON push button switch SW2, shown
in FIG. 2. The contacts 104 are closed to apply power to the
appliance initially. The second set of contacts 106 are also
normally open and are part of a control relay K1 as will be
described herein. As illustrated in FIGS. 2B and 4A (as well as the
alternative embodiment of FIG. 4B), the relay K1 (which includes a
coil 132 and contacts 106) can for convenience be mounted on the
printed circuit board 140 as part of the electronics components
package in the schematic box 101.
A full wave bridge rectifier 114 is provided in parallel with
heating element 43a which is the lower wattage (e.g., 80 W) element
to provide a suitable tap voltage to the motor 50. The rectifier
114 is conventional in design and consists of four diodes
identified in the schematic as D1, D2, D3 and D4. The bridge 114 is
in parallel with and provides DC power to the motor 50 across a
positive voltage node 116 and a return node 118.
A transistor 120, such as a conventional NPN switching transistor,
receives DC power from the bridge 114 through a half wave
rectifying blocking diode 122, a base drive resistor 124 and a
delay capacitor 126. The transistor 120 collector 120a is connected
in series with an LED type light indicator 128 and a current
limiting resistor 130. The transistor collector 120a is also in
series with the K1 relay coil 132. A filter capacitor 134 and a
free wheeling diode 136 are provided in parallel with the relay
coil 136 to regulate the DC voltage. A second push button type
momentary contact switch 138 is connected in parallel with the
delay capacitor 126. The second switch 138 is normally open.
The components within the box 101 are preferably mounted upon a
printed circuit board 140 which is mounted on a bracket 141 on the
interior side of the base wall 30 so that switches 104 and 138 can
be actuated from the exterior, for example through escutcheon plate
49, as shown in FIG. 2B. LED 128 is also mounted in the escutcheon
plate 49. The resistance and capacitance values set forth on FIGS.
4A and 4B are exemplary only.
In operation, when the power switch SW2 is activated, power is
applied to the heaters 43 and the motor 50. DC voltage is also
applied to turn on the transistor 120 after a short delay. When the
transistor 120 is turned on, it provides current flow through the
lamp 128 and the relay coil 132. The lamp 128 provides a visual
indication that the appliance is on with power applied. With
current through the relay coil 132, the relay contacts 106 close
and the relay K1 operates to maintain power to the heaters 43 and
the motor 50. If either of the thermocouples opens or if the
thermostatic device 108 opens, power is interrupted to the heaters
43 and the motor 50. This loss of power also causes the transistor
to turn off, thus extinguishing the LED 128 and also de-energizing
the relay K1. Power cannot be re-applied to the appliance until the
thermostat 108 is manually reset to a closed condition.
The momentary switch 138 (SW1) can be used as an emergency or
manual over ride control switch. When the switch 138 is activated,
the contacts thereof short out the capacitor 126, which causes the
transistor 120 to turn off. The transistor 120 being off forces the
relay K1 to de-energize, thus opening the relay contacts 106 and
cutting off power to the appliance. Note that the contacts 104 only
momentarily close when the main power switch SW2 is activated,
because the relay K1 is used to latch power on for the appliance.
This allows the appliance to remain in an "on" condition but with
an automatic shut-off function performed by the relay K1 in the
event of any electrical overload condition such as motor failure,
bearing failure, or locked rotor.
This circuit further provides the advantage of supplying rectified
AC voltage to the motor 50, to allow the use of a permanent magnet
motor with low initial torque which will not break the magnetic
coupling between the driving and driven magnet arrays. This is a
distinct advantage over applying a pure DC signal to the motor 50
which, with most relatively small magnet motors, would decouple the
magnet arrays, or require the use of a larger motor with larger
magnets.
With reference to FIG. 4B, an alternative control circuit 100 is
provided that allows for a low voltage operation (wherein like
components are provided the same reference numeral as from FIG.
4A). In this embodiment, the relay 132 is connected to a driver
circuit 300 that includes a series pass transistor 302 having a
collector connected to the cathode of the blocking diode 122 and an
emitter connected to the relay K1 coil 132. This transistor
configuration is in the form of a series pass regulator. A zener
diode 304 is connected between the transistor 302 base and ground,
and a resistor pair 310, 312 provide voltage to the transistor 302
base and the cathode of the zener 304. The zener operates to limit
the voltage applied to the relay coil 132. The lamp 128 is provided
in parallel with the relay coil and is lit when power is
applied.
In operation, when the ON switch 104 is actuated, power is applied
from the supply 102 to the heater coils 43, the motor 50 through
the bridge 114, and the relay driver circuit 300. The transistor
302 turns on in response to the voltage level at the cathode of the
diode 122, which in normal or typical operation will produce about
10 volts on the base of the transistor 302 by operation of the
zener diode 304. This causes the transistor 302 to operate
essentially in a saturated condition, with about 9 volts applied to
the relay coil 132. This circuit configuration provides lower
sensitivity to line voltage as compared to the circuit of FIG. 4A
as the relay 132 will pull in even at low line voltages; whereas in
some cases the circuit of FIG. 4A may not provide sufficient power
to activate the relay 132 depending on the heater 34 and motor 50
load.
In the control circuit of FIG. 4B, the anode side of the half wave
rectifying blocking diode 122 is connected to the heater 43 tap as
opposed to the plus side of the motor 50 (as compared to the
configuration of FIG. 4A). This improves the performance of the
circuit by preventing back EMF from the motor 50 from feeding into
the control circuit 300. In the embodiment of FIG. 4A it is
possible in some applications that the back EMF from the motor 50
can prevent the circuit 300 from turning off the relay 132 when the
switch 138 is actuated relatively quickly. By removing the
influence of the back EMF, the embodiment of FIG. 4B operates to
disable the relay 132 even with a fast momentary operation of the
pushbutton switch 138.
* * * * *